Fig. 34.2 ). Reabsorption of NaHCO3 and Na+–organic solutes across the proximal tubule establishes a transtubular osmotic gradient (i.e., the osmolality of the interstitial fluid bathing the basolateral side of the cells is a few mOsm/L higher than the osmolality of tubule fluid) that provides the driving force for the passive reabsorption of water by osmosis. Because more water than Cl− is reabsorbed in the first half of the proximal tubule, the [Cl−] in tubular fluid rises along the length of the proximal tubule (see
Fig. 34.2
In the second half of the proximal tubule, Na+ reabsorption is largely accompanied by Cl− reabsorption via both transcellular and paracellular pathways (
Fig. 34.3 ). Na+ is primarily reabsorbed with Cl− rather than organic solutes or HCO3 − as the accompanying anion because the Na+ transport mechanisms in the second half of the proximal tubule
There are over 300 different solute transporter genes that form the so-called SLC (solute carrier) family of genes.AD, autosomal dominant; AR, autosomal recessive; TAL, thick ascending limb of Henle’s loop; XLR, X-linked recessive.Modified from Nachman RH, Glassock RJ. NephSAP 2010;9(3).
differ from those in the first half, and because the tubular fluid that enters the second half contains very little glucose or amino acids. In addition the high [Cl−] (140 mEq/L)
Fanconi syndrome, a renal disease that is either hereditary in tubule fluid, which is due to preferential reabsorption − or acquired, results from an impaired ability of the proximal of Na+ with HCO3 and organic solutes in the first half − tubule to reabsorb HCO3, Pi, amino acids, glucose, and of the proximal tubule, facilitates reabsorption of Cl− low-molecular-weight proteins. Because other downstream with Na+ . nephron segments cannot reabsorb these solutes and
The mechanism of transcellular Na+ reabsorption in the protein, Fanconi syndrome results in increased urinary excretion of HCO3− , amino acids, glucose, Pi, and low- second half of the proximal tubule is shown in
Fig. 34.3 molecular-weight proteins.